1. Field of the Invention (Technical Field)
The invention relates to a convective air flow calorimeter and a method for its use.
2. Background Art
Human Calorimeters, by Webb, Praeger Publishers (1985), details and classifies a variety of devices for measuring the heat given off by an animal body. Such devices have previously been classified as direct (measuring entire heat loss) or indirect calorimeters, where only respiratory gas exchange was measured. Further, calorimeters have been classified by principle of operation, including flow (both air and water) calorimeters, gradient calorimeters, storage calorimeters, and calorimeters with compensating heaters.
In all cases, of course, the law of conservation of energy is maintained: EQU Q.sub.FD +Q.sub.ST =Q.sub.HL +Q.sub.WK +Q.sub..DELTA.H +Q.sub.FE +Q.sub.UR( 1)
where Q is energy quantity in food (FD), body fuel stores (ST), total heat loss (HL), external work (WK), changes in body heat content (.DELTA.H), feces (FE), and urine (UR).
In calorimetry, energy equation (1) is generally supplanted by the following: EQU M=.+-.R.+-.C.+-.K+E+W.+-.S (2)
where M is heat production from metabolism; R, C, and K are heat exchanges by radiation, convection and conduction, respectively (or sensible heat); E is evaporative heat loss (or insensible heat); W is external work; and S is heat storage in the body.
The first air flow calorimeter was described in "Nouvelle Chambre Calorimetrique du Laboratoire de Bioenergetique," Comptes Renders des Seances de la Societe de Biologie et de ses Filiales, by A. August and J. LeFevre, Vol. 100 (1929). This calorimeter was described by the authors as a totally adiabatic and waterproof chamber wherein an "almost imperceptible" isothermal current of cold air was evenly distributed throughout the chamber by a series of orifices. The air was recirculated, cooled, heated, and purified. Air mass was measured, as was temperature increase. A small portion (20%) of water, carbon dioxide, and intenstinal gases was absorbed and measured. Condensation in the chamber was prevented by regulating the hygrometric state of the chamber. An endometer also provided respiratory and calometric measurements. An ergometer was provided to measure subject work.
"An Isothermic, Gradient-Free, Whole-Body Calorimeter for Long-Term Investigation of Energy Balance in Man", Tschegg et al., Metabolism, Vol. 28, No. 7, July, 1978, basically describes a modern form of the calorimeter described by Auguet and LeFevre. It is noted that the Tschegg et al. calorimeter is an open (not circulatory) system, and the subject is unshielded from such open system flow.
Recent air flow calorimeters include those of Dauncey (Webb, supra, p. 95, et seq.) and Garby (Webb, supra, p. 126, et seq.). Dauncey's device, however, although denominated an air flow calorimeter, actually measures sensible heat as the change in water temperature across a heat exchanger.
Garby's air flow calorimeter apparently distributes circulating air by means of orifices in overhead pipes. Significantly, neither Dauncey nor Garby effectively shield the subject from recirculating air.
A linear disk-type calorimeter is described in American Journal of Physics Teachers,"A Linear Fick's Law Calorimeter," Vol. 50, No. 10, pp. 889-893, by Alpert, et al. (October 1982). Employing selectively heat-conductive metals, such as magnesium, the calorimeter described was a small, simple device suitable for measuring heat loss of small animals. Advantageously, this device has a fast response time.